In the process of manufacturing a variety of semiconductor devices, a silicon oxide film or a silicon nitride film is formed to be used in various applications, e.g., as a gate insulating film of a transistor. As for a method for forming the silicon oxide film or the silicon nitride film, there has been proposed a method of depositing a silicon oxide film or a silicon nitride film by CVD (Chemical Vapor Deposition) as well as a method of forming a silicon oxide film or a silicon nitride film by oxidizing or nitriding silicon through the use of plasma processing, which is disclosed in, e.g., Japanese Patent Laid-open Application Nos. 2000-260767A and 2003-115587A.
Since the step of oxidizing or nitriding silicon by a plasma is repeatedly performed in the process of manufacturing semiconductor devices as described above, it is typical that silicon (silicon layer) used as an object of oxidization or nitriding is mixed on a substrate together with a material film such as a silicon oxide film or a silicon nitride film formed in previous steps. If oxidization or nitriding is carried out on the mixed subsistence of different films, a previously formed silicon nitride film (Si3N4) can be oxidized into a silicon oxynitride film by subsequent plasma oxidization processing and, similarly, a previously formed silicon oxide film (Si3O2) may be nitrided into a silicon oxynitride film by subsequent plasma nitriding processing.
If, however, the material film other than target silicon is oxidized or nitrided in the semiconductor device manufacturing process, an etching selection ratio of the material film becomes different from those of other films in case when the material film is etched away in a subsequent step, thereby resulting in an undesirable effect, e.g., an increase in the number of steps or deterioration of yield.